Maytansine is a compound of the ansa macrolide class, isolated by Kupchan and his co-workers from the East African plants Maytenus serrata and Maytenus buchananii. It shows significant cytotoxicity against the murine leukemia lines L1210, L5178Y, P388, and has antitumor activity against leukemia P388. Studies are being carried out on the effects of maytansine as an inhibitor of macromolecule synthesis, and on its synergism and cross-resistance with other antitumor agents. Its embryotoxic effects and ability to produce mitotic arrest in tumor cells are being studied by a variety of cytological and biochemical techniques. Such studies will be extended to other compounds of the same class as they become available.